Electrochemical Quartz Crystal Nanobalance Analyses of Electrocatalysis for Fuel Cells

为了设计高性能的燃料电池电催化剂，用电化学石英晶体纳米天平(EQCN)分析了铂-铁和铂-钴合金上的电极反应，并与纯铂上的电极反应进行了比较。我们得到了以下结果：1.我们用专门设计的电化学池成功地构建了10 MHz-EQCN系统，并证实了最终的精度为±0.1 Hz(±0.4 ng/cm~2)，并且在长时间内具有很高的稳定性。结果表明，在0.1MHClO_4溶液中，在多次重复的电位扫描过程中，铂-铁和铂-钴电极通过形成少量的铂表层，即合金组分的溶解和薄的铂层的重排而得到电化学稳定。X射线光电子能谱、电感耦合等离子体发射光谱和电感耦合扫描隧道显微镜的结果补充了EQCN的结果。用EQCN分析了铂-铁和铂-钴合金电极的CO耐受性。在共吸附(θ_&lt；Co&gt；=1)的纯铂a-…电极上的正向电位扫描对于更多的铂-铁或铂-钴合金，在0.3~0.5V有一个预氧化峰(P_1)，而在0.7V有一个主要的CO氧化峰(P_2)。每个P_1分别对应于弱吸附的CO在铂-铁和铂-钴合金上的氧化和在纯铂上吸附的羧酸盐的氧化。这与我们以前的原位FTIR结果是一致的，并且有力地支持了我们提出的由纯铂的电子结构改进的铂皮肤的电子结构。在P_2区，CO被H_2O放电生成的OH氧化，CO或CO相关物种(COOH)氧化形成的空位被水合ClO_4阴离子部分占据。用EQCN分析了氧饱和0.1MHClO4溶液中铂、铂-铁和铂-钴电极上的氧还原反应。在纯铂中，即使在高过电压下，仍有恒定量的反应中间体如铂氧(约20%)吸附在表面上。相反，这种中间体在铂-铁和铂-钴电极上的含量很低，可能是由于较快的ORR动力学所致。合金与纯铂之间的这种差异可能归因于铂表面电子结构的改变。较少

In order to find a clue for a design of high-performance electrocatalysts for fuel cells, we analyzed electrode reactions on Pt-Fe and Pt-Co alloy in comparison with that on pure Pt by using an electrochemical quartz crystal nanobalance (EQCN). We obtained the following results.1. We succeeded to construct 10MHz-EQCN system with a specially designed electrochemical cell, and confirmed the ultimate accuracy of ±0.1 Hz (±0.4 ng/cm^2) and high stability among long time.2. It was found that the Pt-Fe and Pt-Co electrodes were electrochemically stabilized by a formation of a few monolayers of Pt-skin layer during several repetitive potential sweeps in 0.1 M HClO_4 solution, i.e., a dissolution of alloy components followed by a rearrangement of thin Pt layer. The results obtained by XPS, ICP and EC-STM complement the EQCN results.3. A CO-tolerance at Pt-Fe and Pt-Co alloy electrodes was analyzed by the EQCN. In a positive-going potential scan at CO-adsorbed (θ_<co>=1) electrodes of pure Pt a … More nd Pt-Fe or Pt-Co alloy, a pre-oxidation peak (P_1) was observed at 0.3 to 0.5 V vs. RHE before a major CO oxidation peak (P_2) at 0.7 V. Each P_1 was assigned to an oxidation of weakly-adsorbed CO at Pt-Fe and Pt-Co alloys and adsorbed carboxylate at pure Pt, respectively. This is consistent with our previous results by in-situ FTIR, and strongly supports a modified electronic structure of the Pt-skin from that of pure Pt, which has been proposed by us. At P_2 region, CO is oxidized by OH formed by H_2O discharge and vacant sites on both of the electrodes formed by the oxidation of CO or CO-related species (COOH) are partly occupied by hydrated ClO_4 anions.4. Oxygen reduction reactions (ORR) at Pt, Pt-Fe, and Pt-Co electrodes in O_2-saturated 0.1 M HClO_4 solution were analyzed by the EQCN. At pure Pt, a constant amount of reaction intermediate such as Pt-O (ca. 20 %) always adsorbed on the surface even under high overvoltage. In contrast, the amount of such an intermediate was found to be fairly low at Pt-Fe and Pt-Co electrodes, probably due to a fast ORR kinetics. Such a difference between alloys and pure Pt may be ascribed to modified electronic structure of the Pt-skin. Less